This invention relates to modules adapted for secure mounting and retention on an object. An example of such an assembly is an electronic sensor and/or control module including an accelerometer and acceleration signal processing apparatus designed to sense the acceleration of the object for the control of a system on the basis of the sensed and processed acceleration signal. Such modules are, for example, mounted on vehicle members in vehicle seat occupant sensing and/or characterization systems for controlling the deployment of passive vehicle restraint devices such as airbags in a vehicle crash. A module used in this manner may include apparatus for sensing vertical acceleration of a seat occupant in an occupant weight sensing system to assist in determining to what extent an indicated occupant weight is actually due to the downward force exerted by a tightly cinched seat belt. Such modules may also, or alternatively, be mounted on vehicle body members for sensing their horizontal accelerations and/or to provide a crash sensing signal.
If a nodule including an accelerometer in such a system is not securely fixed to the object on which it is mounted, the accelerometer may not share the true acceleration of the object, particularly in high accelerations; and the output acceleration signal may thus not accurately represent the acceleration of the object. A system for detecting a secure mounting of the module provides a higher degree of confidence in the reliability of such an accelerometer signal.
In a prior art publication, Research Disclosures No. 453067, published January 2002, an apparatus is described as shown in FIG. 1-3 herein. A case 10 of a module has a main portion 14 and one or more mounting portions 12. A fastener 20 is shown as a bolt having a head 22 and a cylindrical, threaded shank 24. Shank 24 is inserted through a mounting opening 11 in mounting portion 12 and threadably engages an aligned opening 17 in object 18, which may be a member of a vehicle. Head 22 provides an interface for a torque generating tool to threadably advance shank 24 into object 18. Case 10 is made of an electrically non-conducting material such as a thermoplastic resin, and object 18 is made of an electrically conducting material such as steel or aluminum. An electrically conducting insert 26 is molded into case 10 but projects out of and lays on an upper surface 19 of mounting portion 12 around the mounting opening 11. Electrically conducting insert 26 is prevented from lateral contact with shank 24 of fastener 20, if necessary by an insulating sleeve, not shown.
Electrically conducting insert 26 further contacts an additional fastener 28, which secures one end of a circuit board 16 to case 10 within its main portion 14. An electronic circuit includes a circuit trace 36, shown only schematically in FIG. 3 but preferably provided on the underside of circuit board 16, with a portion of the circuit trace surrounding an opening in circuit board 16 through which fastener 28 projects. Fastener 28, in its securely fastening position, contacts both the circuit trace 36, through its head on the underside of circuit board 16, and electrically conductive insert 26, for example through a stepped diameter portion of its shank as shown, to provide electrical contact between insert 26 and circuit trace 36. For this reason, fastener 28 is shown schematically as a switch 28 connecting conductors 26 and 36 in the circuit diagram of FIG. 3.
An electrically conducting washer 30 is provided with fastener 20. This washer surrounds shank 24 directly underneath head 22 and has a frustro-conical shape. As seen in FIG. 1, wherein the apparatus is shown before case 10 is securely fastened to object 18, washer 30 is undistorted: with an outer circumferential edge contacting surface 19 of mounting portion 12 radially outside electrically conducting insert 26 so that it is not in electrical contact therewith. In torqued advancement of fastener 20 downward through mounting portion 12 into object 18, head 22 of fastener 20 provides a downward force on the inner edge of washer 30 to progressively distort washer 30 in a flattening manner until the washer physically and electrically contacts the exposed portion of electrically conducting insert 26, as shown in FIG. 2. This contact establishes a series electrical circuit from object 18, through fastener 20 and washer 30 to electrically conducting insert 26, and continuing through fastener 28 to circuit trace 36 on circuit board 16. FIG. 3 shows this arrangement schematically, with a circuit trace representing object 18 being connected to circuit trace 36 in series through (1) a switch representing fastener 20 and washer 30, (2) a circuit trace representing electrically conducting insert 26, and (3) a switch representing fastener 28. Object 18 is typically grounded to the vehicle chassis; and circuit board 16 further includes other electrical components providing a ground detection circuit, in which a resistor R2, circuit trace 36 and the two switches (20,30 and 28) described above are connected in series between a voltage reference VREF and ground (object 18), while a resistor R1 is connected between circuit trace 36 and an input of a microcomputer 40 programmed with an algorithm to detect by the presented input voltage whether both of the switches are closed.
The operation of the arrangement is based on the fact that a secure mounting of case 10 of the module on object 18 requires a specified clamping force exerted by one or more of the fasteners 20 on its respective mounting portion 12; and achievement of each specified clamping force may be inferred from the distortion of the respective washer to contact the respective electrically conductive insert 26 and thus effectively “close” switch 20,30 (providing switch 28 is also “closed”).
But this prior art arrangement has a disadvantage, in that a normal, flat washer may be substituted for the frustro-conical washer, particularly in a repair or replacement activity. Electrical contact in such case may be achieved between fastener 20 and washer 30 with a minimal clamping force that does not guarantee a secure mounting of the module, and the design specifications on which the validity of the accelerometer signal would no longer apply. It would be desirable to modify this prior art arrangement to prevent such an occurrence.